Today, it is a very common to apply onto the hull, of a ship and the like an antifouling coating composition comprising an organic or inorganic antifouling agent and a resinous binder such as vinyl resin, alkyd resin and the like.
With these, antifouling paints the antifouling effect is fully dependent on the antifouling agent being dissolved out of the coating, and the dissolution of said agent is primarily determined by the diffusion of the agent based upon the concentration gradient of said agent in the coating. As a result, one is unable to obtain a long lasting, stable antifouling effect using these paints.
Furthermore, since the water insoluble resinous component will, after dissolution of said agent from the coating, form a skeleton structure, there are additional disadvantages associated with these paints such as an increase in friction between the ship surface and water, a reduced speed, an increase in fuel consumption and the like. Under these circumstances, an antifouling coating composition comprising an antifouling agent and a hydrolyzable resin vehicle capable of forming a comparatively tough coating which is gradually decomposed by hydrolysis in sea water has become the focus of much research.
The present inventors have already found that a class of polyester resins having a number of metal-ester bonds in their polyester backbone chains are useful as a resinous vehicle in a polishing type antifouling paint, and these resins are the subjects of Japanese Patent Application Nos. 165922/81 and 196900/83.
Such resins have the nature of being easily hydrolyzed, under weak alkaline condition as in sea water at the metal-ester bonding portions, disintegrated into a number of small, low molecular weight segments and dissolved in sea water. However, said resins are primarily of comparatively low molecular weight (e.g. up-to about 2000) and are poor in film-forming property, and therefore, said coating compositions suffer from cracks and peeling of the formed coatings.
If the molecular weight of said polyester resin is increased to a moderate level, it is indeed possible to improve the film-forming property, but, at that time, it will necessarily have a marked decrease in the hydrolysis property of the resin. To compensate for this if the number of metal-ester bonds in the backbone chain of the resin is increased the resultant resin is only soluble in a polar solvent and not in most solvents commonly used in a coating composition, and the formed coating is swollen with sea water. These attempts would therefore, give no fruitful results, and thus, these resins are unsatisfactory for use in antifouling paints.
An attempt has also been made to use a resin whose side chain has a trialkyl tin ester portion as a terminal group. In this type of resin, polarity of the resin is gradually increased in proportion to the progress in hydrolysis of said ester portion, and the resin is finally dissolved in sea water.
Typical examples of such resins are acrylic resins having as a constituent element tri organo tin salts of .alpha., .beta.-unsaturated basic acids. In this case, in order to obtain a stabilized, tough coating, the resin should preferably be free from hydrophilic groups if circumstances allow, and in order to ensure the dissolution of the hydrolyzed resin in sea water, the resin should preferably have as many hydrophilic groups as possible, i.e. more than a certain critical range, after said hydrolysis.
Therefore, in the preparation of such resin by the copolymerization of tri organo tin salt of .alpha., .beta.-unsaturated basic acid and other acrylic vinyl monomers, attempts have been made such that the former is present in a higher concentration in the reaction system and the latter is selected from members with no or the least amount of hydrophilic groups. Thus, a copolymer of acrylate, acrylamide, styrene and the like containing 55 to 70 wt % of tri organo tin salt of .alpha., .beta.-unsaturated monobasic acid has been prepared and can be practically used.
In this type of resin, differing from the aforesaid polyester resin having metal-ester bonds in its main chain, hydrophilic carboxyl groups are generated at the time when the tri organo tin portions at the side chains are released through hydrolysis and the resin is only dissolved in sea water at the stage where the concentration of said carboxyl groups get to a certain critical point. The film-forming property of the resin is also excellent. However, this resin has the disadvantage that a considerable quantity of highly expensive and toxic organo tin compounds are essential. Therefore, from both an environmental and economic point of view, it has been long desired to reduce or eliminate the amount of such material.
In addition to the foregoing, the discoveries present inventors have also discovered (as disclosed in Japanese Patent Application No. 106434/85) a hydrolyzable resin composition which has an excellent film-forming property and whose resin is characterized by having at the side chain portions a particular group capable of resulting a hydrophilic group through hydrolysis, being hydrolyzed and dissolved in sea water at an appropriate rate, and being prepared without the necessity of using a tri organo tin compound which is expensive and toxic, the composition consisting essentially of a resin having at least one side chain bearing at least one terminal group of the formula: ##STR1## M is a metal selected from zinc, copper and tellurium; x is an integer of 1 to 2; R represents an organic acid residue selected from ##STR2## and R.sub.1 is a monovalent organic residue.
Said resin composition is prepared by either one of the following methods.
(1) A mixture of (a) a metal oxide, hydroxide, sulfide or chloride, (b) a monovalent organic acid or its alkali metal salt, and (c) a polymerizable unsaturated organic acid or its alkali metal salt is heated under stirring at a temperature lower than the decomposition temperature of the desired metal ester product, and the by-produced substances as alkali metal chloride, water monovalent organic acid metal ester, bifunctional polymerizable unsaturated organic acid metal salt and the like are removed to obtain a purified metal ester between the polymerizable unsaturated organic acid and the monovalent organic acid.
The thus obtained metal ester between the polymerizable unsaturated organic acid and the monovalent organic acid or the mixture of said metal ester and the monovalent organic metal ester is then subjected to a homopolymerization or a copolymerization with other copolymerizable monomer(s) to give the desired resin having at least one side chain bearing at least one metal ester containing terminal group.
(2) A mixture of (d) a resin having at a side chain an organic acid or its alkali metal salt, (e) a metal oxide, hydroxide, sulfide or chloride, and (f) a monovalent organic acid is heated under stirring at a temperature lower than the decomposition temperature of the desired metal ester containing resin, and the by-product substances are removed, if desired, to obtain a resin having at least one side chain bearing the desired metal ester terminal group.
(3) Alternatively, the desired product may be prepared by reacting a resin having at a side chain an organic acid (g) and a monovalent organic acid metal ester (h) at a temperature of not higher than the decomposition temperature of the desired product, thereby effecting an ester exchange reaction between the materials used.
However, various unsolved questions are still remaining in these methods. That is, in the first method, the selected metal compound happens to exert a polymerization inhibiting activity requiring a quantity of initiator to be present in the reaction system to accelerate the reaction smoothly.
Furthermore, certain monomers are hardly soluble in a conventional reaction solvent and in such case, the reaction must be carried out under heterogeneous reaction conditions or in a considerable quantity of solvent, giving polymers with low molecular weights.
In the second method, since the reaction involved is a neutralization reaction, there is a considerable difficulty in the control of the reaction speed and in the extreme case, there occurs gelation of the reaction product.
And, in the third method, since the reaction involved is a kind of equilibrium reaction and no particular regard is given to removing the by-product monobasic acid out of the reaction system, such acid always remain in the product in the form of free acid and causes blisters in the coating when exposed to ionic atmosphere. Furthermore, such acid may be reacted with an antifouling agent in a coating composition under preparation and storage conditions, causing a remarkable decrease in the quality of the coating composition or the coating prepared therefrom. Free acid may also cause corrosion of the base plate.
It is, therefore, an object of the invention to provide a novel method for the preparation of a metal containing resin composition which is totally free from the abovesaid problems. An additional object of the invention is to provide a method in which a variety of metals may be freely used and such metal can be incorporated in a resin in various forms bonded with carboxylic acid, sulfonic acid and phosphoric acid.
A further object of the invention is to provide a method by which a high boiling organic basic acid can be introduced in side chains of a resin in metal ester form and in a higher reaction yield.
Yet another object of the invention is to provide an antifouling paint which is excellent in film-forming property, and capable of forming a coating without blistering and being excellent in polishing and antifouling effects.